This invention relates to an optical gas sensor.
It has been proposed to sense the presence or absence of a gas or gases by Means of the observation of the change in certain optical properties of thin films. Such gas sensors will herein be termed xe2x80x9coptical gas sensorsxe2x80x9d.
Hitherto, there have been problems in such optical gas sensors (a) because the magnitude of the observable change has been very low, in other words the sensitivity of the device is low or (b) because the rate of change in the relevant optical property such as reflectance or transmittance is slow, giving the detector a slow response, or (c) because the device structures or signal extraction or processing techniques required to monitor the optical changes are complex or expensive to implement.
Materials used in optical gas sensors make a, major contribution to the above factors and have resulted in one or a combination of the problems indicated. Further, after detection of a gas occurs, there has been difficulty in returning the sensor to its original condition, that is the sensor has a slow recovery time.
Some prior art gas sensors of a non-optical type, such as those using tin oxides have had to operate at elevated temperatures which is wasteful of power and could potentially cause flammable gases to ignite. Tin oxide sensors also utilise electrical responses which require electrical connections to the sensor element and thus again constitute a potential hazard. For these reasons, an optically interrogated gas sensor is preferred in many environments.
In a paper, (L. S. Miller, A. L. Newton, C. G. D. Sykesud and D. J. Walton 1991: Optical gas sensing using L B films, Sensors: technology, systems and applications, pp139-143, Adam Hilger (ed K T V Grattan), there is described the use of thin organic films deposited on substrates or on other thin film systems by the Lansgmuir-Blodgett technique, which produces films with a degree of molecular order not normally obtained by other methods and which also leads to reproducible film thicknesses. One such system used an organic film on a silicon substrate. Another utilised a silicon dioxide layer between the silicon and the organic film. The device operated by monitoring the reflectance, preferably of polarised light, from the surface of the layer structure, which is influenced by the optical properties of all the layers and the substrate. The most significant material property of the silicon was its high real refractive index and the most significant material property of the silicon dioxide layer was its lack of absorption and its real refractive index comparable to that of the organic layer, these factors influencing the reflectance obtained because that depends on multiple-beam interference phenomena within the layered structure. In addition, these substrate materials were chosen because they are substantially unreactive to many gases to be sensed.
It has elsewhere been proposed to use reflectance in MOS (Metal Oxide Semiconductor) systems. Interference phenomena may also be utilized to maximise the optical reflectance.
The gas sensing responses are limited by the properties of the thin film materials. Using the techniques outlined in the above-mentioned paper, we have found that phthalocyanines have provided sensitive detection but a very slow response time measured in many minutes. Pyrrole derivatives have provided a quicker response but relatively poor sensitivity.
It is an object of the present invention to provide an optical gas sensor material which has an advantageous combination of high sensitivity and reduced response and recovery times compared with previously proposed materials. It is a further object to provide an optical gas sensor using such a material.
According to the invention there is provided an optical gas sensor comprising an active material deposited on a substrate as a thin film, the active material comprising a highly-polarisable organic material, the sensor being adapted to indicate a change in at least one optical property of the active material due to the presence of a gas.
The active material may be deposited on the substrate by the Langmuir-Blodgett technique.
The highly-polarisable organic material may incorporate a specific electron donating group.
The highly-polarisable organic material may incorporate a specific electron accepting group.
The highly-polarisable organic material may comprise units attached to a polymer backbone.
The polymer backbone may be a polysiloxane.
The active material may be polysiloxane I as defined.
The active material may be an azobenzene derivative.
The active material may be a stilbene derivative.
The sensor may be for detection of a gas comprising one or more oxides of nitrogen, which may be nitrogen dioxide.
The change in optical properties may be a directly visually observable change of colour.
Optical interrogation means may be provided to detect and measure said change in said optical property.
The optical interrogation means may comprise a reflectometer adapted to monitor said change which comprises optical interference effects.
The interrogating optical beam may comprise visible light.
The optical interrogation means may illuminate the film through the substrate.
A conditioning light source may be incorporated for conditioning the active material.
The material may be held at a constant temperature, which may be different from ambient temperature.
The invention also provides a substrate having deposited thereon a thin film of an active material comprising a highly-polarisable organic material for use in an optical gas sensor.
The highly polarisable organic material may comprise an azobenzene derivative which may be a polysiloxane such as Polysiloxane I as herein defined.
The thin film may be deposited by the Langmuir-Blodgett technique.